Steels and alloys intended for high temperature service typically are also subjected to one or more other adverse factors. Thus, each application of such alloys often dictates resistance to two or more of such factors as impact, static or cyclical stresses, thermal shock, thermal fatigue, warping, carburization, corrosion by hot gases, or other substances, and abrasion. Lowest effective cost is also always a very important factor.
For example, portland cement is made from a mixture of limestone, clay, cement rock, or slag, which gives approximately the following composition: 63% calcium oxide, 23% silica, 8% alumina, and the balance small amounts of oxides of iron, magnesium, sulfur, sodium and potassium. The limestone, which is mainly calcium carbonate, and the other substances are heated in a refractory lined rotary kiln to temperatures as high as about 3000.degree. F. (1650.degree. C.). Carbon dioxide is driven out of the carbonate by the intense heat, leaving mainly calcium oxide in a sticky mass of silica and other oxides called clinker. The hot clinker is dumped out of the kiln at about 2500.degree. F. (1370.degree. C.) onto a traveling series of cement cooler grates, usually with a recycled layer of cool, crushed clinker between the grates and the fresh hot clinker. The grates themselves may reach temperatures in some operations as high as 1500.degree. F. to 1600.degree. F. (815.degree. C. to 870.degree. C.) but most likely operate at about 1200.degree. to 1400.degree. (650.degree. C. to 760.degree. C.). Cooling air is passed upwardly through the clinker bed, carrying the hot corrosive gases with it. The grates are thus subjected to the very abrasive clinker and the heat from the clinker due to conduction and radiation effects. Impact and other stresses are not severe.
Enormous tonnages of alloys for the manufacture of these grates are employed around the world, and they must be of relatively low cost. Therefore, nickel-base and cobalt-base superalloys and other exotic alloys are far too expensive for this application. On the other hand, several standard heat resistant alloy types, such as the standard stainless steels and heat resistant alloys of the Alloy Castings Institute Division of the Steel Founders Society of America, have been employed in this application. However, with their usual carbon contents of about 0.40% to 0.75%, those alloys only have a room temperature hardness of around 200 Brinell Hardness Number (BHN) which is lowered to about 80 to 110 BHN at 1300.degree. F. (700.degree. C.), an average working temperature for cement cooling grates. Thus, when the carbon content of process returns or of recycled stainless steel scrap climbs too high, the scrap must either be repurified by a very expensive process to once again reduce carbon, or the too-high-carbon stainless steel may be recycled in nickel-chromium heat resistant alloys which allow much higher carbon contents. As a result, types CF 8M, 316, 316L, 317, 317L and similar stainless steels must either undergo expensive repurification to remove carbon or be employed in some new manner. Furthermore, most of the heat resistant alloys specify a maximum of 0.5% Mo. The use of process returns and recycled scrap from cast or wrought molybdenum-containing stainless steels, which contain from 2% to 4% Mo, presents an additional problem.
Hardness values of various substances are obtained by pressing various types of indenter into their surfaces under standardized loads. However, in the case of all but the simplest solid solution metallic alloys, because the penetration of the indenter is resisted by one or more microscopically small hard particles imbedded in the alloy matrix, the effect of the indenter is to give a sort of average value for the various microscopic grains and particles of different hardnesses encountered under the relatively large area of the indenter. Nonetheless, the derived hardness measurements are found to be quite useful in evaluating properties of metallic alloys.
Hardnesses of the different types of alloy matrices as well as those of various secondary particles and phases not only vary considerably between themselves at room temperature but also possess different rates of softening, coalescing or even disappearing, with increasing temperature. In general, the higher hardness of the additional phases in metallic alloys as well as greater volume fractions of those phases at operating temperatures have pronounced effect in increasing the hot hardness and abrasion resistance of alloys. Contrariwise, factors that increase alloy matrix hardness generally have much less direct effect upon hot hardness and hot wear resistance of an alloy. However, such factors may have the indirect effect of reducing solubility, coalescence or transformation of the harder phases at operating temperatures.
Factors that increase the hot strength of alloys also increase hot hardness. On the other hand, factors that primarily increase hot hardness often reduce hot strength considerably. Hot strength is basically a tensile property, while hot hardness and abrasion resistance are compressive properties.
It is very difficult to formulate in bulk the various secondary phases common to the alloys of interest in order to determine the true hardness values of the phases themselves. However, reasonable estimates of alloy hardness values can be made. For example, the hardness values given in Table I below were taken primarily from "Temperature Dependence of the Hardness of Secondary Phases Common in Turbine Bucket Alloys," J. H. Westbrook, pp. 898-904, Journal of Metals. July 1957, Transactions of the American Institute of Metallurgical Engineering. An additional source of information and verification is Tverdost' Spravochnik. A. A. Ivan'ko, Akademiya Nauk Ukrainskoi SSR, Institute Problem Materialovedeniya, Naukova Dumka, Kiev, 1968. (Hardness Reference Book, A. A. Ivan'ko Academy of Sciences of the Ukrainian SSR, Institute of Materials Science, Science Press, Kiev, 1968.)
The hot hardnesses in Westbrook's work were obtained by special equipment employing a modified Vickers hardness tester with diamond indenter and are reported as the Vickers Hardness Number (VHN). The Vickers hardness of a substance is very close to the Brinell hardness of that substance.
TABLE I ______________________________________ VICKERS HARD- NESS NUMBER AT ROOM AT TEMPER- 1300.degree. F. SUBSTANCE ATURE (700.degree. C.) ______________________________________ ALUMINUM OXIDE 2085 1700 IRON-MOLYBDENUM 1350 1150 ALUMINUM NITRIDE 1230 1070 CHROMIUM CARBIDE 1000 830 IRON-MOLYBDENUM-CHROMIUM 1070 800 SIGMA PHASE IRON-CHROMIUM SIGMA PHASE 1150 680 IRON CARBIDE 565 400 NICKEL TITANIUM GAMMA 500 275 PRIME NICKEL ALUMINUM GAMMA 300 240 PRIME MARTENSITIC 410 STAINLESS 130 50 STEEL AUSTENITEC 316 STAINLESS 130 STEEL ______________________________________
The microstructure of nickel-base superalloys consists of an austenitic, or face-centered-cubic (FCC), solid-solution matrix, very small amounts of carbides, and the coherant intermetallic phase known as gamma prime. Gamma prime precipitates are FCC compounds of the A.sub.3 B type, in which the A atoms are mainly nickel, while the B atoms are mainly aluminum and titanium. These alloys usually contain about 1% to 6% Ti, and their matrices remain fully austenitic. These alloys derive their hot strength and hot hardness from the gamma prime phase.
From Table I it is evident that at both room temperature and the average cooler grate operating temperature of 1300.degree. F. (700.degree. C.), titanium-rich gamma prime is harder than aluminum-rich gamma prime but that neither is as hard at either temperature as any of the other hardening components.
Also shown in Table I are the hardness values of the low-carbon 12% Cr type 410 martensitic stainless steel and of low-carbon 18% Cr-8% Ni-2.5% Mo 316 type austenitic stainless steel. These two examples indicate the very low hot hardness values of both matrix types in the absence of hardening components.
Nickel-base and cobalt-base superalloys are hardened to some extent by large quantities of such solid-solution hardening elements as tungsten, molybdenum, columbium (niobium) and tantalum, but the former derives its main strengthening and hardening from the finely-dispersed gamma prime particles while the latter derives these properties mainly by finely dispersed complex-carbide particles.
Representative examples of prior art which describe alloys said to possess primarily high temperature strength and corrosion resistance include the following patents.
Anger, U.S. Pat. No. 2,857,266, teaches the use of molybdenum and low carbon and aluminum contents in alloys of relatively high nickel and chromium content to improve hot strength and corrosion resistance to about 2300.degree. F. (1260.degree. C.). These alloys depend upon a stable FCC matrix structure for hot strength and have hot hardnesses very close to those of the standard HK type alloys.
Eiselstein et al, U.S. Pat. No. 3,930,904, discloses alloys containing 0.05% to 0.5% Al and 5% to 7% Mo in low-carbon 40% Ni and 15% Cr alloys. The very high nickel content offsets the sigma-forming tendency of the molybdenum and aluminum, and minor quantities of carbides are formed. Such alloys have relatively low hardnesses at all temperatures.
Goda et al, U.S. Pat. No. 3,811,875, teaches 0.25% to 2% Al and up to 3.5% Mo additions in low-carbon austenitic stainless steels. Their room temperature hardness (Table I) ranges from 78 to 86 Rockwell B, which is equivalent to about 140 to 166 BHN, respectively. Their hardness at 1300.degree. F. (700.degree. C.) would therefore be expected to be about 90 to 100 BHN.
Yamaguchi et al, U.S. Pat. No. 4,141,762, teaches 0.06% to 6% Al and up to 6% Mo in low-carbon duplex alloys of mixed FCC-BCC matrix structures said to have remarkably excellent hot workability. The alloys are formulated to be of high ductility and low hardness, as contrasted to high hardness and no significant hot workability.
Wick, U.S. Pat. No. 3,167,424, teaches alloys for valve seat insert castings of 0.85% to 3.50% C., 1.5% to 3.5% Si, 0.5% to 3% Mn, 4% to 12% Mo, 4% to 12% Co, 4% to 12% W, 0.2% to 6% Cu, 0.2% to 4% Al, 5% to 35% Fe, 5% to 20% Cr and the remainder Ni. The preferred composition is 2.5% C, 2.5% Si, 1% Mn, 40% Ni, 10% Cr, 7% Mo, 7% Co, 7% W, 1.5% Cu, 1.5% Al and 20% Fe. While such alloys might have reasonably good hot hardness at 1300.degree. F. they are entirely beyond the cost range of cement cooler grate castings due to their nickel, cobalt, tungsten and molybdenum contents.
In spite of these prior art efforts, there remains a need for low cost alloys having good high temperature hardness and resistance to hot corrosion gases.